Implantable medical devices (IMDs) are used to treat patients suffering from a variety of conditions. Examples of IMDs involving cardiac devices are implantable pacemakers and implantable cardioverter-defibrillators (ICDs). Such electronic medical devices generally monitor the electrical activity of the heart and provide electrical stimulation to one or more of the heart chambers when necessary. For example, pacemakers are designed to sense arrhythmias, i.e., disturbances in heart rhythm, and in turn, provide appropriate electrical stimulation pulses, at a controlled rate, to selected chambers of the heart in order to correct the arrhythmias and restore the proper heart rhythm. The types of arrhythmias that may be detected and corrected by IMDs include bradycardias (unusually slow heart rates), which can result in symptoms such as fatigue, dizziness, and fainting, and certain tachycardias (unusually fast heart rates), which can result in sudden cardiac death (SCD).
Implantable cardioverter-defibrillators (ICDs) also detect arrhythmias and provide appropriate electrical stimulation pulses to selected chambers of the heart to correct the abnormal heart rate. In contrast to pacemakers, however, an ICD can also provide pulses that are much stronger and less frequent. This is because ICDs are generally designed to correct fibrillation, which is a rapid, unsynchronized quivering of one or more heart chambers, and severe tachycardias, during which the heartbeats are very fast but coordinated. To correct such arrhythmias, ICDs deliver low, moderate, or high-energy shocks to the heart.
Generally, IMDs are equipped with an on-board memory in which telemetered signals can be stored for later retrieval and analysis. In addition, a growing class of cardiac medical devices, including implantable heart failure monitors, implantable event monitors, cardiovascular monitors, and therapy devices, can be used to provide similar stored device information. Typically, the telemetered signals can provide patient physiologic and cardiac information. This information is generally recorded on a per heartbeat, binned average basis, or derived basis, and involve, for example, atrial electrical activity, ventricular electrical activity, minute ventilation, patient activity score, cardiac output score, mixed venous oxygen score, cardiovascular pressure measures, time of day, and any interventions and the relative success of such interventions. Telemetered signals can also be stored in a broader class of monitors and therapeutic devices for other areas of medicine, including metabolism, endocrinology, hematology, neurology, muscular disorders, gastroenterology, urology, ophthalmology, otolaryngology, orthopedics, and similar medical subspecialties.
Generally, upon detecting arrhythmias and, when necessary, providing corresponding therapies to correct such arrhythmias, the IMDs store the telemetered signals over a set period of time (usually before, during, and after the occurrence of such arrhythmic event). Current practice in the art involves the use of an external communication unit, e.g., an external programmer, for non-invasive communication with IMDs via uplink and downlink communication channels associated with the communication device. In accordance with conventional medical device programming systems, a programming head can be used for facilitating two-way communication between IMDs and the external communication device. In many known implanted IMD systems, the programming head can be positioned on the patient's body over the IMD side site such that one or more antennae within the head can send RF signals to, and receive RF signals from, one or more antennae disposed within the hermetic enclosure of the IMD or disposed within the connector block of the IMD in accordance with common practice in the art.
Implementation and operation of most, if not all, RF communication systems for IMDs and external communication devices involves a balancing or compromising of certain countervailing considerations, relating to such interrelated operational parameters as data transmission rate and transmission range, among numerous others. Such operational parameters are often interrelated in the sense that the adjustment of one operating parameter may permit or require the adjustment of one or more other operating parameters even while predetermined system performance goals and/or requirements continue to be met and predetermined limitations imposed upon operational parameter adjustment are adhered to. One example of this is the trade-off between signal range and signal power. Simply stated, for a given communication scheme, a more powerful (e.g., higher amplitude) signal has a longer effective range. Thus, decreasing the range of a communication link (e.g., reducing the distance between transmitters and receivers in the link) allows the transmission power to be decreased, while other operational parameters, e.g., data transmission rate, can be held at a constant value.
Another example is the trade-off between data transmission rate and transmitted signal power. Those of ordinary skill in the art will appreciate that in most instances, increasing the data transmission rate over an RF channel typically requires increased signal bandwidth. Increasing the bandwidth, in RF, tends to lead to increased power consumption by the communication system in order to maintain an acceptable signal-to-noise ratio.
Still another example of the trade-offs associated with the operational parameters, and system performance goals of an RF communication system is associated with data transmission rate versus signal range. As noted above, increasing data transmission rate typically results in an increased bandwidth of the transmitted signals; conversely, decreasing data transmission rate typically reduces signal bandwidth. If bandwidth can be reduced, the range of operation can be increased for a given level of power consumption.
As noted above, the foregoing and other trade-offs associated with various operational parameters of a communication system arise in most applications involving RF transmission and reception. However, the nature of the interrelation between the various operational parameters may vary depending, for example, upon the type of modulation used (e.g., pulse position modulation, frequency shift keying, frequency modulation, amplitude modulation, etc.), as well as upon the type of coding used. In the context of IMD systems, there are additional considerations that must be addressed. Primary among these are reliability of transmission and reception, and conservation of implanted device power. Conservation of implanted device power (which in most cases implies minimization of current drain upon an implanted device's internal battery) in particular renders the aforementioned trade-offs—rate-versus-range, range-versus-power, rate-versus-power, as well as others—highly significant. In most cases, however, the settings of operational parameters of interest are static, or if adjustable, are adjusted simply using a single parameter.